1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling a magnetic disc drive for performing a record on a magnetic disc.
2. Description of the Related Art
Corresponding to a desire to both downsize and a larger capacity of a magnetic disc unit in recent years, a recording density on a magnetic disc has been enhanced, i.e., a track density and a bit density have been enhanced. Enhancing the bit density entails the use of a magnetoresistance head (MR head) capable of taking a greater level of a regenerative signal as a reading head in place of an inductive head capable of effecting conventional read/write processing. For this reason, there are prepared two pieces of magnetic heads, i.e., the inductive head for writing and the MR head for reading.
Track-directional positions of these two heads are different from each other, and, hence, a variety of contrivances are needed for the control method.
FIG. 24 is an explanatory diagram of the MR head FIG. 25 is an explanatory diagram of a yaw angle. FIG. 26 is an explanatory diagram of a data format of the magnetic disc.
As illustrated in FIG. 24, a magnetic head 90 has an MR head 90-2 used as a read head in order to enhance the bit density. When this MR head is employed as the read head 90-2, it is required that a write head 90-1 be separately provided. For example, an inductive head is employed for the write processing. A variety of problems to be obviated arise when using such an MR head.
First, the heads 90-1, 90-2 of the magnetic head 90 have different gap positions. On the other hand, a rotary type actuator is employed for moving the magnetic head 90 in radial directions of the magnetic disc. This rotary type actuator rotates about the center of rotation, and its locus depicts a circular arc. Accordingly, an angle (known as a yaw angle) to a track (cylinder) direction of the magnetic head is not 0.degree..
Besides, as shown in FIG. 25, the yaw angle changes on the inner and outer sides of the magnetic disc. For this reason, especially a track positional deviation of the MR head 90-2 is induced. Therefore, a mixing degree of signal components of adjacent tracks fluctuates depending on the respective tracks. This leads to a decline in terms of resolution of the read data.
For preventing this decline, there was proposed a method of reducing a gap width of the MR head 90-2 so that the gap of the MR head 90-2 does not extend over to the adjacent track on any track. Based on this method, however, there arises such a problem that a read output level decreases, and, correspondingly, an S/N ratio also increases. For this reason, a core width of the read head 90-2 is narrowed only to some extent. Even though done in such a way, when an offset is caused due to a thermal off-track or the like, a positional margin on one side is sufficient, but the positional margin on the opposite side is reduced.
As a method of increasing this positional margin, there is proposed a yaw angle correcting method of changing offset quantities both in a read position and in a write position (see Japanese Patent Laid-Open Publication No.4-232610).
As illustrated in FIG. 26, generally in a data format of the magnetic disc, an ID part is provided in front of a data part. Accordingly, when executing a write instruction, the ID part is read, and the writing to the data part is carried out. Hence, according to the prior art, the ID part is read, and, therefore, a seek with a correction of the yaw angle is effected. Made thereafter is a seek movement corresponding to a correction quantity of the yaw angle, thus effecting the writing process to the data part.
Second, the MR head 90-2 is defined as a magnetoresistance element. This MR head 90-2 reads the data by flowing a bias current. That is, the data is read by making use of variations in resistance value according to a magnetic force of the magnetoresistance element. This MR head 90-2 has a portion where a resistance variation with respect to the magnetic force does not exhibit a linear characteristic. For this reason, an operating point is set at a portion where the resistance variation with respect to the magnetic force takes the linear form by regulating the bias current. This regulation has hitherto been done manually.
Third, the MR head 90-2, as explained above, reads the data by running the bias current. Therefore, a voltage is applied to the MR head 90-2. Hitherto, the voltage has been applied to the MR head 90-2 simultaneously when switching ON a power supply.
Fourth, when enhancing the bit density, servo data on a servo surface of the magnetic disc are recorded with a high density by use of the MR head. Consequently, even a trace of defect of the servo data is precisely read.
There exist, however, the following problems inherent in the prior art.
According to the conventional yaw angle correcting method of changing the offset quantities in the read and write positions, the seek with the correction of the yaw angle is executed when performing the writing process, and the ID part is read. Thereafter, the writing to the data part is carried out after being moved corresponding to the correction quantity of the yaw angle, and hence this takes a time, correspondingly. Therefore, as illustrated in FIG. 26, it is required that a gap (GAP)-2 between the ID part and the data part be elongated, resulting in a reduction in capacity of the data part on the magnetic disc.
Further, in the conventional manual regulation of the bias current, the bias current is manually regulated while seeing a regenerative waveform through an oscilloscope or the like. It is therefore difficult to set an optimal bias current. This conduces to a decrease in read margin.
Moreover, the voltage has hitherto been applied to the MR head 90-2 simultaneously with the power-ON, and, hence, the voltage was applied before the magnetic disc was rotating sufficiently. For this reason, it follows that the voltage is applied in the course of floating of the MR head 90-2. This brings about the possibility in which the MR head 90-2 is damaged due to discharging with respect to the magnetic disc having no electric potential. In addition, because the voltage is applied when not required, and consequently a decline of the characteristic due to variations with a passage of time is caused in the MR head 90-2.
Similarly, when the defect occurs in the servo data, the servo control is not well conducted, resulting in a destruction of the data or a stoppage of the control system.